>>21132017if you orbit a spinning black hole, you progressively pick up speed due to frame dragging, and this is acceleration without feeling the g forces. you could fling yourself to any other part of the universe with a black hole (to do the reverse there to decelerate)
fission / fusion / antimatter drives could definitely work, the astronauts would live under a constant 5g acceleration (with intermittent breaks), and travel 40 light years in what feels to them like 5.5 years (2.75 years to reach 20 light years, then decelerate for the rest of the journey)
